Magnetic tunnel junction (MTJ) devices are comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and are based on the phenomenon of spin-polarized electron tunneling. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ device a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed as memory cells for use in nonvolatile magnetic random access memory (MRAM) arrays. In an MRAM the resistance of the MTJ memory cell is lowest when the magnetic moments or magnetization directions of both ferromagnetic layers are parallel, and is highest when the magnetic moments are antiparallel. The basic structure of an MTJ memory cell is described in IBM's U.S. Pat. No. 5,650,958, which also describes an MTJ memory cell wherein one of the ferromagnetic layers has its moment fixed or pinned by being exchange coupled with an antiferromagnetic layer and the other ferromagnetic layer is free to have its moment rotated parallel or antiparallel to the moment of the fixed ferromagnetic layer in the presence of an applied magnetic field.
MTJ devices have also been proposed as magnetic field sensors, such as magnetoresistive read heads in magnetic recording disk drives, as described in IBM's U.S. Pat. No. 5,898,548. In MTJ read heads the fixed ferromagnetic layer typically also has its moment pinned by an antiferromagnetic layer and oriented substantially perpendicular to the surface of the magnetic layer on the disk. However, unlike in an MRAM, in the absence of an applied magnetic field the moment of the free ferromagnetic layer is oriented substantially perpendicular to the moment of the fixed ferromagnetic layer, i.e., substantially parallel to the surface of the magnetic layer on the disk.
In MTJ read heads, it is possible that during the lapping process to form the air-bearing surface (ABS) of the air-bearing slider, the material from the ferromagnetic layers may smear at the ABS and short out across the tunnel barrier layer. In addition, many antiferromagnetic materials and the aluminum oxide typically used for the tunnel barrier can also corrode during the ABS lapping process. For this reason a flux-guided MTJ read head has been proposed, as described in IBM's U.S. Pat. No. 5,898,547, which is incorporated herein by reference. In the '547 patent, only the free ferromagnetic layer is exposed at the ABS, with the tunnel barrier layer, the fixed ferromagnetic layer and the antiferromagnetic layer having their edges recessed from the ABS.
FIG. 1 shows the prior art flux-guided MTJ read head of the '547 patent. The MTJ 100 includes the antiferromagnetic layer 116, the fixed ferromagnetic layer 118, the tunnel barrier layer 120 and the free ferromagnetic layer 132. The fixed ferromagnetic layer 118 is the bottom ferromagnetic layer and is formed on the antiferromagnetic layer 116. The tunnel barrier layer 120 is formed on the fixed layer 118 and the free ferromagnetic layer 132 is the top ferromagnetic layer and thus formed on the tunnel barrier layer 120. The free ferromagnetic layer 132 has its sensing edge 140 substantially coplanar with the sensing surface or ABS. However, the other layers of the MTJ 100 have their front edges recessed from the ABS. The free layer 132 thus serves both as the sensing ferromagnetic layer for sensing data from the magnetic layer on the disk and as a flux guide for directing the magnetic flux back to the tunnel barrier layer 120 of the MTJ 100. The layers 102, 104 are the top and bottom electrical leads, respectively, for the head. The bottom lead 102 is formed on the G1 gap layer of the head and the G2 gap layer separates the top lead 104 from magnetic shield S2. The material of G1 and G2 is an electrically insulating material, typically alumina. Alternatively, the bottom lead 102 may be formed directly on the first magnetic shield S1 and the second magnetic shield S2 may be formed on the second lead 104. The shields S1 and S2 are formed of Ni—Fe alloys or Ni—Fe—Co alloys and are electrically conducting, so that an electrically conductive path is provided through the shield S1 to bottom lead 102, perpendicualarly through the MTJ 100 to top lead 104 and the second shield S2. If the leads 102, 104 are in direct contact with shields S1, S2, respectively, then insulating material is still required at the front and back edges of the MTJ 100. The sensing surface or ABS may have a protective overcoat formed on it, such as a thin layer of amorphous diamond like carbon, as is known in the art to protect the head during contact with the disk.
A flux-guided MTJ read head as shown in FIG. 1, wherein the free ferromagnetic layer is on top, is not the most desirable type of structure because it requires deposition of the free layer following formation of the recessed layers in the MTJ, which causes a poor interface to the tunnel barrier or a flux guide which is too thin to be useful. The problem with forming the free ferromagnetic layer on the bottom is that the tunnel barrier layer becomes rough when the bottom free layer is grown on the conventional underlayer, typically Ta, which makes it difficult to form the tunnel barrier layer thin enough. In addition, it is desirable to position the free layer closer to the center of the shield-to-shield gap to avoid diversion of the signal flux into the shields. However, it is known that increasing the Ta underlayer thickness to achieve this free layer positioning does not improve the smoothness of the tunnel barrier layer.
What is needed is an MTJ device and in particular a flux-guided MTJ read head that does not suffer from the problems associated with having the free layer on the bottom.